Ink-jet printing is a technique which puts a number of constraints on the nature of the ink which is used. These constraints include low viscosity (usually less than 50 cP or, for continuous operation, less than 10 cP at 25.degree. C.), sufficient surface tension that the jet can produce the necessary large number of small droplets, and (at least for continuous operation) sufficient conductivity that the droplets can be directed as desired onto a substrate to be printed. The ink must be a homogeneous liquid capable of rapid conversion to a dry printed area on the substrate.
Hitherto, these constraints have been satisfactorily met by the use of ink-jet inks comprising a binder, a colourant, a conductive component and, as a major component of the liquid phase, low molecular weight organic solvents. In order to obtain the desired combination of solubility and drying characteristics, a mixture of several solvents may be used but, in order to achieve rapid drying, they are highly flammable, volatile solvents such as methyl ethyl ketone and ethanol.
Known ink-jet inks may be UV-curable. For example, the binder component is a pre-polymer, usually of high functionality.
The concept of piezoelectric ink-jet printing is based on the movement of a piezoelectric ceramic transducer when a voltage is applied to it. When the voltage is applied, the length of the ceramic decreases, creating a void which is filled with ink. When the voltage is removed, the ceramic expands to its full length and the excess ink is repelled, ejecting a drop of ink from the printhead. A fresh drop of ink is expelled on demand.
Inks for use in piezoelectric drop-on-demand ink-jet printing traditionally comprise a mixture of fatty acids, fatty esters and oil-soluble dyes. Such inks should be slow-drying, to prevent clogging of the nozzles; this constraint results in the process being suitable only for surfaces where the ink may dry through absorption On non-absorbent surfaces, the ink remains wet for an undesirably long period of time.
U.S. Pat. No. 4,303,924 discloses a radiation-curable ink-jet ink comprising, in addition to a colourant and a conductive component, 0 to 90% of monofunctional polymerisable monomer and 5 to 80% of multifunctional polymerisable monomer. The colourant is a dye. The conductive component is oil-soluble. It is indicated that there may be 0% organic solvent, but all the given Examples contain more than 10% of such a solvent which, together with the high content of trifunctional monomer, provides a liquid phase in which the conductive component is soluble. If such solvent were absent, there would be no solvent for the conductive component, and it is unlikely that the viscosity of the ink would be sufficiently low for ink-jet printing. Further, the presence of non-polymerisable organic solvent such as methanol or ethanol presents the problem that the ink may dry in an open nozzle. The ink must apparently be used in an inert gas atmosphere, which is a considerable disadvantage.
European Patent Publication No. 407054 discloses an ultra-violet jet ink comprising a curable adhesive thinned with solvents which, at least in the Examples, invariably include at least the organic solvent methyl ethyl ketone. This composition also presents the problem that the ink may dry in the nozzle.
An object behind the present invention has been to provide an ink-jet ink which meets the given constraints but which avoids the use of volatile, flammable, environmentally-undesirable solvents, and can be used in air.